System and method for reselecting antennas in a cellular mobile communication system using multiple antennas

ABSTRACT

A cellular mobile communication system using multiple antennas reselects a plurality of transmission antennas of base stations connected to a mobile station. The system includes measuring a channel quality parameter used for estimation of channel performance, determining whether to reselect the transmission antennas using the measured channel quality parameter, reselecting transmission antennas of the base stations connected to the mobile station, and connecting the reselected transmission antennas to the base station.

PRIORITY

This application claims priority under 35 U.S.C. § 119 to an applicationentitled “Partial Handoff in MIMO-OFDM Cellular Systems” filed in theUnited States patent and Trademark Office on Apr. 14, 2004 and assignedSer. No. 60/561,891, and an application entitled “System and Method forReselecting Antennas in a Cellular Mobile Communication System UsingMultiple Antennas” filed in the Korean Intellectual Property Office onJan. 7, 2005 and assigned Ser. No. 2005-1674, the contents of both ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a technology for reselectingan antenna in a cellular mobile communication system, and in particular,to a technology for reselecting a plurality of antennas of base stationsconnected to a mobile station as the mobile station shifts from oneplace to another.

2. Description of the Related Art

The conventional mobile communication system provides a voice-orientedservice, and chiefly depends on channel coding in order to overcomedeterioration in channel quality. However, due to an increase in demandfor high-quality multimedia service that enables users to communicatewith anyone, anytime, anywhere, the conventional voice service isevolving into a data service and there is a demand for a next generationradio transmission technology for transmitting the increased amount ofdata at a higher data rate and at a lower error rate. In particular,importance of high-speed data transmission in a forward link having thegreater amount of required data is ever increasing.

A mobile communication environment deteriorates the reliability of thesignals due to fading, shading effect, propagation attenuation, noise,and interference. A fading phenomenon due to multiple paths brings aboutsignificant signal distortion caused by the sum of signals havingdifferent phases and sizes, which are received through different paths.The fading effect is one of the difficulties that should be overcome inorder to achieve a high-speed data communication, and extensiveresearches is being conducted to overcome the wireless channelcharacteristics and utilize the features. As a result, a Multiple InputMultiple Output (MIMO) technology using a plurality of transmission andreception antennas has been proposed.

In a cellular mobile communication system using the multiple antennas,unlike in the conventional cellular mobile communication system, amobile station and a base station, both of which use a plurality ofantennas, can exchange data with each other at a higher speed withoutincreasing a bandwidth of the system.

Even in the cellular mobile communication system using multipleantennas, like in the conventional cellular mobile communication systemin which a mobile station uses a single antenna, performance of achannel formed between a mobile station and a base station varies due tothe movement of the mobile station, and a need for forming a new channelto another antenna occurs as the mobile station moves from one cell toanother, or if channel quality is to be adjusted. The cellular mobilecommunication system using multiple antennas, as a base station has aplurality of antennas even in a single cell, should occasionallyreselect antennas according to time-varying channel performance. Forexample, even when a mobile station moves between sectors in a celldivided by the plurality of antennas of the base station, an antennashould be reselected for the same base station.

In addition, when a mobile station moves from a current cell (servingcell) to another cell (target cell), the mobile station in anoverlapping region between the cells may connect with both an antenna ofa serving base station and an antenna of a target base station. In thiscase, it is difficult to reconfigure a channel with the conventionalhandoff scheme.

SUMMARY OF THE INVENTION

The present invention provides a system and method for reselecting aplurality of antennas of base stations connected to a mobile stationaccording to channel performance in a cellular mobile communicationsystem using multiple antennas.

The present invention provides a system and method in which a mobilestation is simultaneously connected to base stations of neighbor cellsusing a plurality of antennas when the mobile station moves betweencells in a cellular mobile communication system using multiple antennas.

The present invention provides a system and method for reselecting aplurality of antennas of base stations connected to a mobile stationduring a handoff in a cellular mobile communication system usingmultiple antennas.

The present invention provides a system and method in which a mobilestation is simultaneously connected to neighbor base stations using aplurality of antennas in a region where a plurality of cells overlapeach other during a handoff in a cellular mobile communication systemusing multiple antennas.

According to one aspect of the present invention, there is provided amethod for reselecting an antenna for a data channel by a mobile stationin a cellular mobile communication system including the mobile stationhaving at least two antennas, a home base station having a plurality ofantennas, at least one of which is used to form the data channel to themobile station, and at least one neighbor base station for the home basestation, the method including the steps of determining whether toreselect antennas according to channel qualities individually measuredfor first antennas of the home base station and second antennas of theneighbor base station; and selecting at least two antennas from amongthe first antennas and the second antennas when it is necessary toreselect the antennas.

According to another aspect of the present invention, there is provideda method for reselecting an antenna for a mobile station by a home basestation used to form a data channel to the mobile station having atleast two antennas in a cellular mobile communication system usingmultiple antennas, the method including the steps of receiving from themobile station channel qualities individually measured for firstantennas of the home base station and second antennas of a neighbor basestation for the home base station; determining whether to reselectantennas for the mobile station according to the channel qualities; andselecting at least two antennas from among the first antennas and thesecond antennas when it is necessary to reselect the antennas.

According to further another aspect of the present invention, there isprovided a system for reselecting an antenna for a data channel by amobile station in a cellular mobile communication system including themobile station having at least two antennas, a home base station havinga plurality of antennas, at least one of which is used to form the datachannel to the mobile station, and at least one neighbor base stationfor the home base station, the system including the mobile station fordetermining whether to reselect antennas based on channel qualitiesindividually measured for first antennas of the home base station andsecond antennas of the neighbor base station, selecting at least twoantennas from among the first antennas and the second antennas based onthe measured channel qualities when it is necessary to reselect theantennas, and transmitting information on the selected at least twoantennas to the home base station; and the home base station forreceiving information on the measured channel qualities and transmittinga command for selection of at least the two antennas to the mobilestation according to the received information.

According to further another aspect of the present invention, there isprovided a system for reselecting an antenna for a mobile station by ahome base station used to form a data channel to the mobile stationincluding at least two antennas in a cellular mobile communicationsystem using multiple antennas, the system including the mobile stationfor measuring channel qualities for first antennas of the home basestation and second antennas of a neighbor base station for the home basestation, and transmitting the measured channel qualities; and the homebase station for determining whether to reselect antennas for the mobilestation based on the measured channel qualities, and selecting at leasttwo antennas from among the first antennas and the second antennasaccording to the measured channel qualities when it is necessary toreselect the antennas.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a diagram illustrating a cellular mobile communication systemusing multiple antennas to which the present invention is applied;

FIG. 2 is a diagram illustrating a structure of a mobile stationaccording to a first embodiment of the present invention;

FIG. 3 is a diagram illustrating a basic concept of a handoff in acellular mobile communication system using multiple antennas to whichthe present invention is applied;

FIG. 4 is a diagram illustrating a method for determining whether toreselect antennas according to an embodiment of the present invention;

FIG. 5 is a diagram illustrating a structure of a base station accordingto the first embodiment of the present invention;

FIG. 6 is a signaling diagram illustrating a method for reselectingtransmission antennas in a cellular mobile communication system usingmultiple antennas according to the first embodiment of the presentinvention;

FIG. 7 is a diagram illustrating a structure of a mobile stationaccording to a second embodiment of the present invention;

FIG. 8 is a diagram illustrating a structure of a base station accordingto the second embodiment of the present invention; and

FIG. 9 is a signaling diagram illustrating a method for reselectingtransmission antennas in a cellular mobile communication system usingmultiple antennas according to the second embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Several preferred embodiments of the present invention will now bedescribed in detail with reference to the annexed drawings. In thefollowing description, a detailed description of known functions andconfigurations incorporated herein has been omitted for conciseness.

The present invention reselects a plurality of antennas of base stationsconnected to a mobile station in a cellular mobile communication systemusing multiple antennas. For convenience of description, it will beassumed herein that a base station serves as a transmission side and amobile station serves as a reception side, that a forward link having agreater amount of required data is taken into consideration, and thusthe antennas of the base station are referred to as transmissionantennas, and antennas of the mobile station are referred to asreception antennas. However, it should be noted that the presentinvention is not limited to the forward link.

As illustrated in FIG. 1 in a cellular mobile communication system usingmultiple antennas to which the present invention is applied includesmobile stations (MSs) 12 and base stations (BSs) 11 each of whichinclude multiple antennas. A channel of the cellular mobilecommunication system using multiple antennas is formed with acombination of a plurality of reception antennas of the mobile stations12 and a plurality of transmission antennas of the base stations 11. Inparticular, as the mobile stations 12 include a plurality of receptionantennas, a plurality of reception antennas included in the same mobilestation 12 can be simultaneously connected to transmission antennas ofdifferent base stations 11.

It is general provision that the mobile stations 12 continuously shiftfrom one place to another, channels between the plurality of antennas ofthe mobile stations 12 and the base stations 11 should be changed uponoccasions for the best channel performance. In particular, the cellularmobile communication system using multiple antennas, as a base stationhas a plurality of transmission antennas even in a single cell, shouldoccasionally reselect transmission antennas according to time-varyingchannel performance. For example, even when a mobile station movesbetween sectors in a cell divided by the plurality of transmissionantennas of the base station, transmission antennas should be reselectedfor the same base station. In addition, when a mobile station moves froma current cell (serving cell) to another cell (target cell), the mobilestation in an overlapping region (also known as “handoff region”)between the cells can connect with both a plurality of transmissionantennas of a serving base station managing the serving cell and aplurality of transmission antennas of a target base station managing thetarget cell. Of course, even in a non-handoff region, depending uponchannel performance, all of the reception antennas of a mobile stationmay be connected to the transmission antennas of the serving basestation managing the serving cell or to the transmission antennas of thetarget base station managing the target cell. The present invention canbe applied to all of the foregoing cases.

The present invention can be roughly implemented with two methods. Inone method, a mobile station reselects transmission antennas of basestations connected thereto. In another method, a base station reselectstransmission antennas of base stations connected to a mobile station.

Herein, a description of the present invention will be separately madefor a first embodiment in which a mobile station reselects transmissionantennas of base stations connected thereto, and a second embodiment inwhich a base station reselects transmission antennas of base stationsconnected to a mobile station.

In a cellular mobile communication system using multiple antennas, inorder to reselect a plurality of transmission antennas of base stationsconnected to a mobile station according to the present invention, aprocess of measuring a channel quality parameter based on which channelperformance is estimated, and a process of determining whether toreselect transmission antennas using the estimated channel qualityparameter should be initially performed. After performing the process ofmeasuring a channel quality parameter and determining whether toreselect transmission antennas, the system performs a main process ofreselecting transmission antennas of base stations connected to a mobilestation.

First Embodiment

A first embodiment of the present invention provides a system and methodin which a mobile station reselects transmission antennas of basestations connected thereto.

With reference to FIG. 2 a description will be made of a structure of amobile station according to the first embodiment of the presentinvention.

Referring to FIG. 2, a mobile station includes a storage 21, a parametermeasurer 22, a channel performance estimator 23, an antenna reselectcontroller 24, and a feedback signal generator 25.

The storage 21 stores a candidate list of neighbor base stationsneighboring a base station connected to the mobile station in thecurrent position (hereinafter referred to as a “home base station”). Thecandidate list of the neighbor base stations includes identifications(IDs) of the base stations, the number of transmission antennas providedby a corresponding base station, and information related to thetransmission antennas which can be reselected according to the presentinvention. The stored information can be derived from a memory (notshown) of the mobile station or can be received from base stationsthrough control signals (for example, beacon signals).

Further, the candidate list of the neighbor base stations includes theinformation related to the home base station for the following reason.As the base stations include a plurality of transmission antennas, eventhough the particular transmission antennas have poor channelperformance, the other transmission antennas having good channelperformance can be connected to reception antennas of a mobile station,thereby forming high-performance channels. In addition, even though amobile station is located in a handoff region, all of the transmissionantennas selected in a transmission antenna reselecting process can beeither transmission antennas of the home base station or transmissionantennas of the neighbor base stations according to channel performance.

The parameter measurer 22 measures a channel quality parameter, an indexbased on the performance of the channels connected to the home basestation and the neighbor base stations. It is preferable to periodicallyperform the measurement on the channel quality parameter. Alternatively,the measurement on the channel quality parameter can be performed atspecific occasions. For example, the measured channel quality parameterincludes noise power, signal power, correlation feature, Received SignalStrength Indicator (RSSI), Carrier-to-Interference ratio (C/I),Signal-to-Noise Ratio (SNR), and Signal-to-Interference Noise Ratio(SINR).

The channel performance estimator 23 generates information based onwhich performance of respective channels can be estimated, using thechannel quality parameter measured by the parameter measurer 22.

The antenna reselect controller 24 has a threshold value with which itcan determined whether to reselect transmission antennas, for thechannel performance estimation information generated by the channelperformance estimator 23. Based on the threshold value, the antennareselect controller 24 determines whether to reselect transmissionantennas based on whether the channel performance estimation informationis greater or less than the threshold value. If it is determined thatthe transmission antennas should be reselected, the antenna reselectcontroller 24 provides to the home base station the feedback signalgenerator 25 with the measured channel quality parameter value to betransmitted.

If a transmission antenna reselect command is received from the homebase station, the antenna reselect controller 24 reselects thetransmission antennas reconnected to the reception antennas of themobile station by consulting the candidate list of the neighbor basestations, stored in the storage 21. The antenna reselect controller 24transmits the information related to the reselected transmissionantennas and the information on the neighbor base stations including thereselected transmission antennas to the home base station through thefeedback signal generator 25.

In the first embodiment of the present invention, the transmissionantenna reselection is performed by the antenna reselect controller 24of the mobile station, and its preceding process of determining whetherto reselect transmission antennas depending on channel performanceestimation information is also performed by the antenna reselectcontroller 24.

The feedback signal generator 25 quantizes the measured channel qualityparameter value provided from the antenna reselect controller 24,information on the reselected transmission antennas, and information onthe neighbor base stations including the reselected transmissionantennas, and transmits the quantization results to the home basestation.

A detailed description will now be made of a method in which theparameter measurer 22 measures a channel quality parameter, and a methodin which the antenna reselect controller 24 determines whether toreselect transmission antennas, and reselects the transmission antennasaccording to the determination result.

First, the method in which the parameter measurer 22 measures a channelquality parameter will be described. In the following description, itwill be assumed that the parameter measurer 22 measures a channelquality parameter in a MIMO Orthogonal Frequency Division Multiplexing(MIMO-OFDM) cellular communication system. However, the channel qualityparameter measurement method according to the present invention is notrestricted to the MIMO-OFDM cellular communication system, and caninclude all of the channel quality parameter measurement methods for thecellular mobile communication systems using multiple antennas.

Generally, an input/output relation in a time domain in a MIMO-OFDMcommunication system is defined as Equation (1).

$\begin{matrix}{{y\left( {n,p} \right)} = {{\sum\limits_{l = 0}^{L}{{h\left( {l,p} \right)}\;{x\left( {{n - l},p} \right)}}} + {v\left( {n,p} \right)}}} & (1)\end{matrix}$where {h(l,p)} (where l=0, . . . , L) denotes samples of a MIMOmultipath propagation channel at a time constant ‘n’ for an OFDM symbol‘p’, y(n,p) denotes a vector sampled at an n^(th) sample for the OFDMsymbol ‘p’, and v(n,p) denotes a term for noise and interference for asample ‘n’ and an OFDM symbol ‘p’.

Equation (1) can be expressed as Equation (2) in a frequency domain.Y(f,p)=H(f,p)X(f,p)+V(f,p)  (2)where ‘f’ denotes a tone, i.e., a subcarrier.

In the MIMO-OFDM communication system, noise power is measured asfollows. In the time domain where it is assumed that no signal istransmitted (x(n,p)=0 for all n and a given OFDM symbol p), noise poweris defined as Equation (3).

$\begin{matrix}{\sigma_{v}^{2} \cong {\frac{1}{N\; M_{t}}\;{\sum\limits_{n}{{y\left( {n,p} \right)}}^{2}}}} & (3)\end{matrix}$where σ_(v) is the noise power, the summation is performed over Nsamples and M_(t) is the number of transmission antennas.

Equation (3) can be expressed as Equation (4) in the frequency domain.When the values of the several subcarriers for a subchannel satisfyingX(f,p)=0 are null for a given OFDM symbol p and some of the sets of thesubcarriers f for a subchannel F, noise power is defined as Equation(4).

$\begin{matrix}{\sigma_{v}^{2} \cong {\frac{1}{N\;{F}}{\sum\limits_{f}{{Y\left( {f,p} \right)}}^{2}}}} & (4)\end{matrix}$where the summation is performed over all subcarriers f, and |F| denotesa size of a subchannel.

An average of signal power transmitted in a base station to which thenoise power calculated in accordance with Equation (3) and Equation (4)is given can be measured in the following way.

In the time domain where it is assumed that a signal is transmitted(there is a value of x(n,p) for all n and a given OFDM symbol p), anaverage of signal power is estimated by Equation (5).

$\begin{matrix}{\sigma_{x}^{2} \cong {{\frac{1}{N\; M_{t}}{\sum\limits_{n}{{y\left( {n,p} \right)}}^{2}}} - \sigma_{v}^{2}}} & (5)\end{matrix}$where the summation is performed over N samples.

Equation (5) can be expressed as Equation (6) in the frequency domain.When there is an X(f,p) value for a given OFDM symbol p and some sets ofsubcarriers f for a subchannel F, an average of signal power is definedas Equation (6).

$\begin{matrix}{\sigma_{y}^{2} \cong {{\frac{1}{N\;{F}}{\sum\limits_{f}{{Y\left( {f,p} \right)}}}} - \sigma_{v}^{2}}} & (6)\end{matrix}$where the summation is performed over all subcarriers f, and |F| denotesa size of a subchannel.

A signal-to-noise pulse interference ratio can be calculated using theestimated noise power and signal power.

Some statistical features of channels can be estimated through the pilotinformation, and in this manner, it is possible to estimate the varianceof the channels.

For a channel formed from a transmission antenna #k of a base station toa reception antenna #m of a mobile station, the channel feature in thetime domain is estimated by Equation (7).

$\begin{matrix}{\sigma_{m,k}^{2} \cong {\frac{1}{P}{\sum\limits_{p}{\left\lbrack {h\left( {0,p} \right)} \right\rbrack_{m,k}}^{2}}}} & (7)\end{matrix}$where [h(0,p)]_(m,k) denotes a channel sample in an m^(th) column and ak^(th) row of h(0,0), and its average is calculated for P channelinformation pieces.

Equation (7) can be expressed as Equation (8) in the frequency domain.

$\begin{matrix}{\sigma_{m,k}^{2} \cong {\frac{1}{PF}{\sum\limits_{p,f}{\left\lbrack {H\left( {f,p} \right)} \right\rbrack_{m,k}}^{2}}}} & (8)\end{matrix}$where the summation is performed over P OFDM symbols and F tones, i.e.,F subcarriers.

In addition, a parameter that can be measured by the parameter measurer22 includes a correlation feature. In particular, an average oftransmission correlations indicates a correlation between twotransmission antennas.

In the time domain, a correlation feature between a transmission antenna#k and a transmission antenna #m can be expressed as Equation (9).

$\begin{matrix}{\rho_{m,k}^{2} \cong {\frac{1}{PR}{\sum\limits_{p}{\sum\limits_{r}{\left\lbrack {h\left( {0,p} \right)} \right\rbrack_{r,k}\;\left\lbrack {h\left( {0,p} \right)} \right\rbrack}_{r,m}^{*}}}}} & (9)\end{matrix}$

Herein, the correlation feature is calculated as an average for Rreception antennas and P correlation information pieces, and is averagedas many times as the total number of available multiple paths.

Equation (9) can be expressed as Equation (10) in the frequency domain.

$\begin{matrix}{\rho_{m,k}^{2} \cong {\frac{1}{PRF}{\sum\limits_{f}{\sum\limits_{p}{\sum\limits_{r}{\left\lbrack {h\left( {f,p} \right)} \right\rbrack_{r,k}\;\left\lbrack {h\left( {f,p} \right)} \right\rbrack}_{r,m}^{*}}}}}} & (10)\end{matrix}$where the summation is performed over P OFDM symbols, F tones, i.e. Fsubcarriers, and R reception antennas.

The correlation feature can be significantly affected depending on howthe antennas of the base station are arranged, and as the arrangementbasically fixed, the correlation feature may be pre determined. In thiscase, a base station may transmit the correlation feature information toa mobile station through a control signal, rather than the mobilestation measuring the correlation feature.

The antenna reselect controller 24 determines whether to reselecttransmission antennas in the following method. Because the presentinvention reselects transmission antennas of a base station according tochannel performance as described above, it can be used for all of thecases where there is a change in the channel performance. In thefollowing description, the present invention will be limited to ahandoff situation which most directly shows the case where there is achange in the channel performance.

With reference to FIG. 3, a description will now be made of a basicconcept of a handoff in a cellular mobile communication system usingmultiple antennas to which the present invention is applied.

A mobile station 30 communicates with antennas 31 and 32 of a basestation A using its antennas 38 and 39, in a cell 35. If the mobilestation 30 moves toward a cell 36 and enters a handoff region 37 wherethe cell 35 and the cell 36 overlap each other, a transmission antennareselecting process according to the present invention is performed. InFIG. 3, as a result of performing the transmission antenna reselectingprocess, the antenna 31 of the base station A and an antenna 33 of abase station B are selected as transmission antennas connected to themobile station 30 in the handoff region 37. Therefore, the mobilestation 30 is newly connected to the antenna 31 of the base station Aand the antenna 33 of the base station B in the handoff region 37, andperforms a handoff. Upon leaving the handoff region 37, the mobilestation 30 is connected to the antennas 33 and 34 of the base station Bby re-performing the transmission antenna reselecting process, andperforms communication using the antennas 33 and 34.

In the cellular mobile communication system using multiple antennas,because a mobile station includes a plurality of antennas, a pluralityof antennas of a mobile station can be connected to antennas ofdifferent base stations in a handoff region where cells overlap eachother. Therefore, according to the present invention, a mobile stationcan perform communication seamlessly during a handoff.

However, the method for determining whether to reselect transmissionantennas according to the present invention is not limited to thehandoff situation, and it would be obvious to those skilled in the artthat the restrictive method can be applied to all of the cases wherethere is a change in channel performance.

The antenna reselect controller 24 determines whether to reselecttransmission antennas, using channel performance estimation informationgenerated by the channel performance estimator 23. A description thereofwill be made with reference to FIG. 4.

The channel performance estimation information used herein includes asignal strength indicator (SSI) and a channel correlation indicator(CCI). The SSI is an average value of signal powers, and the CCI is arandom information value capable of representing a channel correlationfeature calculated using the transmission correlation value. Forexample, the CCI can be defined as the maximum transmission correlationvalue, and is set to a value which is less than or equal to 1.

The SSI and the CCI are generated by the channel performance estimator23 and delivered to the antenna reselect controller 24. Morespecifically, the channel performance estimator 23 delivers SSI_(h),SSI_(t), CCI_(h), and CCI_(t) values to the antenna reselect controller24. SSI_(h) is a signal strength value of a channel through which themobile station is connected to the home base station, and SSI_(t) is asignal strength value of a channel through which the mobile station isconnected to a neighbor base station (or target base station). CCI_(h)is a channel correlation value of a channel through which the mobilestation is connected to the home base station, and CCI_(t) is a channelcorrelation value of a channel through which the mobile station isconnected to the neighbor base station.

The antenna reselect controller 24, receiving the SSI_(h), SSI_(t),CCI_(h), and CCI_(t) values, determines the threshold values H_(in) ^(h)and H_(in) ^(t) based on the SSI_(h) and CCI_(h) and determinesthreshold values H_(out) ^(h) and H_(out) ^(t) based on the SSI_(t) andCCI_(t), according to a lookup table. Herein, the threshold value H_(in)^(h) is a threshold value for home base station's signal strength, withwhich the home base station determines that the mobile station moves inthe handoff region as described in connection with FIG. 3, and thethreshold value H_(in) ^(t) is a threshold value for a neighbor basestation's signal strength, with which the neighbor base stationdetermines that the mobile station moves in the handoff region. Thethreshold value H_(out) ^(h) is a threshold value for a home basestation's signal strength, with which the home base station determinesthat the mobile station moves out from the handoff region as describedin connection with FIG. 3, and the threshold value H_(out) ^(t) is athreshold value for a neighbor base station's signal strength, withwhich the neighbor base station determines that the mobile station movesout from the handoff region.

A method for calculating the threshold values will now be described indetail. A threshold value H_(in), with which the home base station andthe neighbor base station determine that the mobile station moves in thehandoff region, and a threshold value H_(out), with which the home basestation and the neighbor base station determine that the mobile stationmoves out from the handoff region, are defined as Equation (11).H _(in) =H _(in) _(—) _(fixed)·(1−CCI·π ₁)H _(out) =H _(out) _(—) _(fixed)·(1−CCI·π ₂)  (11)wherein π₁>1, π₂>1, and the π₁ and π₂ are empirical constants used fordetermining the threshold values. H_(in) _(—) _(fixed) is a fixed signalstrength value used for determining that a mobile station moves in ahandoff region, and H_(out) _(—) _(fixed) is a fixed signal strengthvalue used for determining that the mobile station moves out from thehandoff region. This calculation can be equally applied to the home basestation and the neighbor base station, or can be independently appliedto the home base station and the neighbor base station.

After receiving the SSI_(h), SSI_(t), CCI_(h), and CCI_(t) values andfully determining their corresponding threshold values, the antennareselect controller 24 determines whether to reselect transmissionantennas in the handoff region. Specifically, as illustrated in FIG. 4,if SSI_(t)>H_(in) ^(t) and SSI_(h)<H_(in) ^(h), the antenna reselectcontroller 24 determines that the mobile station is in a handoff state,determining that the mobile station has moved in the handoff region, anddetermines to perform a transmission antenna reselecting processaccording thereto. If SSI_(t)>H_(out) ^(t) and SSI_(h)<H_(out) ^(h), theantenna reselect controller 24 determines that the mobile station is notin a handoff state, determining that the mobile station has moved outfrom the handoff region, and determines to perform a transmissionantenna reselecting process according thereto.

The method in which the antenna reselect controller 24 determineswhether to reselect transmission antennas includes a method that usesthe antenna configuration information between a mobile station and abase station as follows.

A description will now be made of another method for determining whetherto reselect transmission antennas when a mobile station moves in ahandoff region.

The antenna reselect controller 24 performs the measurement needed foran antenna reselecting process. Thereafter, the antenna reselectcontroller 24 calculates the channel performance information L_(current)for the best antenna configuration between the home base station towhich the mobile station is currently connected, and the mobile station.In addition, the antenna reselect controller 24 calculates a channelperformance matrix L_(target)(k) for all of the other antennaconfigurations related to the home base station and neighbor basestations, which can be connected to the mobile station in the handoffregion, and the mobile station. As a result, if max_(k)L_(target)(k)>L_(current)+δ, the antenna reselect controller 24determines that the mobile station is in the handoff state, anddetermines to reselect transmission antennas according thereto. Herein,δ is a threshold value indicating that a mobile station has moved into ahandoff region. The measurement needed for the antenna reselectingprocess performed by the antenna reselect controller 24 is notseparately performed by the antenna reselect controller 24 tosubstantially determine whether to reselect transmission antennas, butis merely provided to describe the method for determining whether toreselect transmission antennas on an overall basis. The measurementneeded for the antenna reselecting process is substantially performed bythe parameter measurer 22, and it is preferable that the antennareselect controller 24 calculates L_(current) and L_(target)(k) usingthe channel quality parameter previously measured by the parametermeasurer 22 only when it is necessary to determine whether to reselecttransmission antennas.

Similarly, a description will now be made of a process of determiningwhether to reselect transmission antennas when the mobile station movesout from the handoff region.

The antenna reselect controller 24 performs the measurement needed foran antenna reselecting process. Thereafter, the antenna reselectcontroller 24 calculates a channel performance matrix L_(current)(k) forthe best antenna configuration between a mobile station and basestations in the current handoff region. In addition, the antennareselect controller 24 calculates the channel performance informationL_(target) for the best antenna configuration between a mobile stationand base stations outside the handoff region. As a result, ifL_(target)>max_(k) L_(current)(k)+δ, the antenna reselect controller 24determines that the mobile station is not in the handoff state, anddetermines to reselect transmission antennas according thereto. Herein,δ is a threshold value indicating that a mobile station has moved into ahandoff region. Like in the case where the mobile station moves into thehandoff region, the measurement needed for the antenna reselectingprocess performed by the antenna reselect controller 24 is notseparately performed by the antenna reselect controller 24 tosubstantially determine whether to reselect transmission antennas, butis merely provided to describe the method for determining whether toreselect the transmission antennas on an overall basis. The measurementneeded for the antenna reselecting process is substantially performed bythe parameter measurer 22, and it is preferable that the antennareselect controller 24 calculates L_(current)(k) and L_(target) usingthe channel quality parameter previously measured by the parametermeasurer 22 only when it is necessary to determine whether to reselecttransmission antennas.

In the method where the antenna reselect controller 24 determineswhether to reselect transmission antennas using antenna configurationinformation between a mobile station and base stations, the antennareselect controller 24 should calculate yields for all of the antennaconfigurations between the mobile station and the base stations whencalculating the channel performance matrix L_(current)(k) and theLcurrent(k). However, in order to reduce the complexity needed for thecalculation, a transmission antenna reselecting process described belowis first performed to change an antenna configuration such that thecalculation can be exclusively performed on the reselected transmissionantennas.

Finally, a description will be made of a method in which the antennareselect controller 24 reselects transmission antennas of base stationsconnected to a mobile station in a handoff region.

A transmission antenna reselecting process reselects optimaltransmission antennas for sets of all of the available transmissionantennas of the neighbor base stations as well as a home base station,to be connected to a plurality of antennas of a mobile station, in thecase where it is necessary to change a channel due to a variation in theperformance of a currently connected channel, including a handoff case.

Assume that a set of reselectable transmission antennas and basestations to which the reselectable transmission antennas belong is S.The antenna reselect controller 24 estimates the performance R(s) foreach transmission antenna s in the set S, and selects a transmissionantenna with max_(s in S)R(s) as an optimal transmission antenna.

For example, when performance of a transmission antenna is estimated inan SNR, the antenna reselect controller 24 determines an SNR,SNR_(i)[k], for a k^(th) tone, i.e. k^(th) subcarrier, and an i^(th)data stream, for a given subset for a transmission antenna s in the S.In addition, the antenna reselect controller 24 selects the optimaltransmission antennas in a ratio of an SNR for each antenna subset inthe S. Herein, an average, a minimum ratio, or an outage ratio can beused for the ratio.

The most important factor in this transmission antenna reselectingprocess is the number of sets that should be searched as the optimalselection. Although the transmission antenna reselecting process selectsa transmission antenna subset having the optimal performance from all ofthe possible transmission antenna subsets, the present invention ischaracterized by reducing the complexity of the transmission antennareselecting process by reducing a size of the subset to be selected. Forexample, the antenna reselect controller 24 selects random transmissionantenna subsets from among the transmission antenna subsets selectablein the handoff region. Alternatively, there is a method for defining thetransmission antenna subsets to be used in the handoff region and forfixing a transmission antenna subset to be selected so that transmissionantennas should be selected with only the corresponding subset. Asanother example, if a neighbor base station supports one data stream,the neighbor base station selects a predetermined transmission antennaor a random transmission antenna, and if a neighbor base stationtransmits a signal with two transmission antennas, it uses twotransmission antennas having the least correlation. Such decorrelatingsubsets can be preset, thereby reducing the number of searchablesubsets. The transmission antenna subset information can be pre storedin the storage 21, can be received from the base stations, or can beexperimentally determined in the mobile station through measurements.

For example, a method for reducing the complexity of the transmissionantenna reselecting process when selecting optimal transmission antennasfrom among the antennas of the base station A and the base station B inthe handoff region described with reference to FIG. 3 can be dividedinto the following four methods. A first method searches for a sethaving the best yield from among all of the possible sets of theantennas of the base station A and the base station B. This methodincludes a case where only the antennas included in the base station Aare selected, and/or another case where only the antennas included inthe base station B are selected. In this method, the transmissionantenna algorithm is most complex but has best performance. A secondmethod unconditionally selects both of the antennas of the base stationA and the antennas of the base station B, and then searches for anantenna set having the best yield. A third method unconditionallyselects both of the antennas of the base station A and the antennas ofthe base station B, and then randomly selects one of the possibleantenna sets. A fourth method unconditionally selects both of theantennas of the base station A and the antennas of the base station B,and previously fixes an antenna set to be used for a handoff.

A description will now be made of a cellular mobile communication systemusing multiple antennas to which the mobile station according to thefirst embodiment of the present invention is applied.

With reference to FIG. 5, a description will first be made of a basestation corresponding to the mobile station according to the firstembodiment.

As illustrated in FIG. 5, a base station includes an antenna reselectscheduler 52, an antenna reselect manager 53, and a channel informationprocessor 54.

The channel information processor 54 determines if there is a change inchannel quality such as an SNR average and data throughput for thechannels, according to a measured channel quality parameter valuereceived from the feedback signal generator 25 of the mobile station.The channel information processor 54 delivers information on thereselected transmission antennas and information on neighbor basestations including the reselected transmission antennas, received fromthe feedback signal generator 25 of the mobile station, to the antennareselect manager 53. When the transmission antennas should bereselected, the channel information processor 54 delivers theinformation on the reselected transmission antennas to the antennareselect scheduler 52.

The antenna reselect manager 53 transmits a transmission antennareselect request to a neighbor base station based on the informationprovided from the channel information manager 54. In response, anantenna reselect scheduler 52 of the neighbor base station determineswhether to approve the transmission antenna reselect request receivedfrom a home base station, and transmits the result to the home basestation. A criterion for the determination includes the system traffic,a service quality request, the reselected antenna, etc.

When transmission antennas should be reselected, the antenna reselectscheduler 52 receives from the channel information processor 54 theinformation on the transmission antennas to be connected to the mobilestation, and allocates corresponding transmission antennas to the mobilestation.

Data from the antenna reselect manager 53 and the antenna reselectscheduler 52 undergoes Medium Access Control (MAC) processing,modulation, multiplexing, Inverse Fast Fourier Transform (IFFT) andguard interval insertion, before being transmitted.

With reference to FIG. 6, a description will now be made of a method forreselecting transmission antennas in a cellular mobile communicationsystem using multiple antennas according to the first embodiment of thepresent invention. FIG. 6 is a signaling diagram illustrating a methodfor connecting two reception antennas of a mobile station 30 toreselected transmission antennas on the assumption that the transmissionantennas reselected in the mobile station 30 including the two receptionantennas include a transmission antenna of a base station A which is ahome base station and a transmission antenna of a base station B whichis a neighbor base station as illustrated in FIG. 3.

In step S61, the mobile station transmits a measured channel qualityparameter value, the information on the reselected transmission antennasand the information on the neighbor base stations including thereselected transmission antennas to the base station A. Herein, becausethe reselected transmission antennas include a transmission antenna ofthe base station A which is a home base station and a transmissionantenna of the base station B which is a neighbor base station, theinformation on neighbor base stations includes information on the basestation A and the base station B.

In step S62, based on the information on reselected transmissionantennas and the information on neighbor base stations includingreselected transmission antennas, the base station A transmits atransmission antenna reselect request to the base station B.

In step S63, the base station B transmits to the base station A anapproval message indicating an approval for the transmission antennareselect request, if it can provide a service with the reselectedtransmission antenna.

In step S64, the base station A forwards data to the base station B uponreceiving the approval message. The forwarded data includes the datathat the mobile station should currently receive through a forward link.The reason for forwarding the data is because as connections between themobile station and the reselected transmission antennas have not beencompleted yet, the other party in communication with the mobile stationtransmits the data only to the base station A. Thus, the base station Ashould forward the data to the base station B in order to enable thebase station B to transmit the data to the mobile station.

In step S65, the base station A transmits a transmission antennareselect command to the mobile station.

In step S66, the mobile station transmits to the base station A and thebase station B information on the transmission antennas reselectedthrough a transmission antenna reselecting process.

In step S67, the base station A and the base station B individuallytransmit data to the mobile station using the corresponding transmissionantennas based on the information on the reselected transmissionantennas. In step S68, the base station A transmits an approval messagefor transmission antenna reselection to the mobile station. Then themobile station releases the connections to the transmission antennas towhich current channels are formed, and forms new channels to thereselected transmission antennas. It is possible to enable parallelconcurrent transmission by using a data channel in step S67 and acontrol channel in step S68.

The mobile station can continuously receive data from the base station Awhich is a home base station even while connecting with the reselectedtransmission antennas.

Second Embodiment

A second embodiment of the present invention provides a system andmethod in which a base station performs a process of reselecting thetransmission antennas to be connected to a plurality of receptionantennas of a mobile station. In particular, the second embodimentprovides a system and method in which a home base station performs atransmission antenna reselecting process.

With reference to FIG. 7, a description will now be made of a structureof a mobile station according to the second embodiment of the presentinvention.

Referring to FIG. 7, a mobile station includes a parameter measurer 71and a feedback signal generator 72.

The parameter measurer 71 measures a channel quality parameter, an indexbased used to estimate the performance of channels connected to the homebase station and the neighbor base stations. It is preferable toperiodically perform the measurement on the channel quality parameter.Alternatively, the measurement on the channel quality parameter can beperformed upon certain occasions. For example, the measured channelquality parameter includes noise power, signal power, correlationfeature, RSSI, C/I, SNR, and SIR. A detailed method for measuring thechannel quality parameter by the parameter measurer 71 is to the same asthe method for measuring the channel quality parameter by the parametermeasurer 22 in the first embodiment, so a detailed description thereofwill be omitted herein.

The feedback signal generator 72 quantizes the channel quality parametermeasured by the parameter measurer 71, and transmits the quantizationresults to the home base station.

With reference to FIG. 8, a description will now be made of a basestation that performs a transmission antenna reselecting process whenthere is a variation in channel performance due to movement of a mobilestation in a cellular mobile communication system using multipleantennas according to the second embodiment of the present invention.

Referring to FIG. 8, a base station includes an antenna reselectscheduler 81, an antenna reselect manager 82, a channel informationprocessor 83, and a storage 84.

The storage 84 stores a candidate list of the neighbor base stations.Because each base station is essentially fixed in its position, thestorage 84 is apt to have information on the neighbor base stations,which is usually not changed. The candidate list of neighbor basestations include the IDs of the base stations, the number oftransmission antennas provided by a corresponding base station, and theinformation on the transmission antennas which can be reselectedaccording to the present invention.

Even in the second embodiment, the neighbor base stations include thehome base station. For example, it is noted from FIG. 3 that the mobilestation 30 communicates with a base station A, which is a home basestation, and a base station B, which is a neighbor base station, in thehandoff region 37. Even though the mobile station is located in thehandoff region, all of the transmission antennas selected in atransmission antenna reselecting process can be either the transmissionantennas of the home base station or the transmission antennas of theneighbor base stations according to channel performance.

The channel information processor 83 generates information based onwhich performance of respective channels can be estimated, using themeasured channel quality parameter value received from the feedbacksignal generator 72 of the mobile station.

The antenna reselect manager 82 has a threshold value used to determinedwhether to reselect transmission antennas, for the channel performanceestimation information generated by the channel information processor83. Based on the threshold value, the antenna reselect manager 82determines whether to reselect transmission antennas based on whetherthe channel performance estimation information is greater or less thanthe threshold value. A process of determining whether to reselecttransmission antennas by the antenna reselect the manager 82 is thatsame as the process of determining whether to reselect transmissionantennas by the antenna reselect controller 24 in the first embodiment,so a detailed description thereof will be omitted herein.

If it is determined that the transmission antennas should be reselected,the antenna reselect manager 82 transmits a transmission antennareselect request to a neighbor base station by consulting theinformation stored in the storage 84. In response, an antenna reselectmanager 82 of the neighbor base station determines whether to approvethe transmission antenna reselect request received from a home basestation, and transmits the result to the home base station. A criterionfor the determination includes the system traffic, a service qualityrequest, a reselected antenna, etc.

Upon receiving an approval for a handoff request from the neighbor basestation, the antenna reselect manager 82 commands the antenna reselectscheduler 81 to reselect transmission antennas to be connected to themobile station.

Then the antenna reselect scheduler 81 reselects the transmissionantennas based on the channel performance estimation informationgenerated by the channel information processor 83 and the candidate listof neighbor base stations. The antenna reselect scheduler 81 transmitsto the corresponding neighbor base station the information on thereselected transmission antennas. A process of reselecting thetransmission antennas by the antenna reselect scheduler 81 is to thesame as the process of reselecting transmission antennas by the antennareselect controller 24 in the first embodiment, so a detaileddescription thereof will be omitted herein.

In response, an antenna reselect scheduler 81 of the neighbor basestation transmits an approval message to the home base station if it canprovide a service with a corresponding transmission antenna indicated bythe information on the selected transmission antennas, received from thehome base station. Thereafter, the antennal reselect scheduler 81 of theneighbor base station allocates the corresponding transmission antennato the mobile station and communicates with the mobile station using theallocated transmission antenna.

In the second embodiment of the present invention, the transmissionantenna reselecting process is performed by the antenna reselectscheduler 81 of the base station and its preceding process ofdetermining whether to reselect transmission antennas is performed bythe antenna reselect manager 82 of the base station.

Data from the antenna reselect manager 82 and the antenna reselectscheduler 81 undergoes MAC processing, modulation, multiplexing, IFFT,and guard interval insertion, before being transmitted.

With reference to FIG. 9, a description will now be made of a method forreselecting the transmission antennas in a cellular mobile communicationsystem using multiple antennas according to the second embodiment of thepresent invention. FIG. 9 is a signaling diagram illustrating a methodfor connecting two reception antennas of a mobile station 30 to thereselected transmission antennas on the assumption that the reselectedtransmission antennas include a transmission antenna of a base stationA, which is a home base station, and a transmission antenna of a basestation B, which is a neighbor base station, as illustrated in FIG. 3.

In step S91, the mobile station transmits the measured channel qualityparameter value to the base station A.

In step S92, the base station A determines whether to reselecttransmission antennas through the antenna reselect manager 82. In stepS93, if it is determined that the transmission antennas should bereselected, the base station A transmits a transmission antenna reselectrequest to the base station B including the reselected transmissionantenna. At this point, because the base station A which is the homebase station also includes the reselected transmission antenna, all ofthe commands, the information and the messages transmitted from the homebase station to the base station B are processed in the home basestation itself. Therefore, this operation will be omitted from thedrawing. The operation is applied in the same way even in the followingprocess.

In step S94, the base station B transmits an approval message indicatingapproval for the transmission antenna reselect request to the basestation A, if it can provide a service with the reselected transmissionantenna.

In step S95, upon receiving the approval message the base station Aforwards data to the base station B. The forwarded data includes thedata that the mobile station should currently receive through a forwardlink. The reason for forwarding the data is because as the connectionsbetween the mobile station and the reselected transmission antennas havenot been completed yet, the other party in communication with the mobilestation transmits the data only to the base station A. Thus, the basestation A should forward the data to the base station B in order toenable the base station B to transmit the data to the mobile station.

The base station A performs a transmission antenna reselect process instep S96, and transmits to the base station B the information on thetransmission antennas reselected through the transmission antennareselect process in step S97.

Then the base station B transmits an approval message for the reselectedtransmission antennas in step S98, and allocates the correspondingtransmission antennas to the mobile station in step S99. Then the mobilestation releases connections to the transmission antennas to whichcurrent channels are formed, and forms new channels to the reselectedtransmission antennas.

The transmission antennas reselected in step S96 include an antenna ofthe base station A which is a home base station. The base station Aprocesses the information on the reselected transmission antennas to beconnected to the mobile station by itself in step S97, and also performsapproval processing on the reselected transmission antennas by itself instep S98.

The mobile station can continuously receive data from the base station Awhich is a home base station even while connecting with the reselectedtransmission antennas.

As described above, the present invention reselects a plurality ofantennas of the base stations connected to a mobile station according tochannel performance in a cellular mobile communication system usingmultiple antennas, thereby maintaining the optimal channel state andthus providing high-quality multimedia service.

In addition, the present invention separates the transmission antennasas far as possible using a plurality of antennas, thereby reducing thecorrelation between the transmission antennas. The reduction in theantenna correlation that considerably affects performance of amultiantenna system and a reduction in the spatial correlation effectcontribute to an increase in system capacity, link throughput, and otherperformances.

While the invention has been shown and described with reference to acertain preferred embodiment thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

1. A method for reselecting an antenna for a data channel by a mobilestation in a cellular mobile communication system that includes themobile station having at least two antennas, a home base station havinga plurality of antennas, at least one of which is used to form the datachannel to the mobile station, and at least one neighbor base station ofthe home base station, the method comprising the steps of: determiningwhether to reselect antennas based on channel qualities of firstantennas of the home base station and second antennas of the neighborbase station; and selecting at least two antennas from among the firstantennas and the second antennas based on the channel qualities; whereinthe mobile station transmits the measured channel qualities to the homebase station, and performs the antenna selecting step in response to acommand from the home base station.
 2. The method of claim 1, whereinthe step of determining whether to reselect antennas comprises the stepof comparing a channel quality for an antenna to which the currentchannel is formed with a threshold value for a channel quality.
 3. Themethod of claim 1, wherein the step of determining whether to reselectantennas comprises the step of comparing a channel quality for anantenna to which the current channel is formed with channel qualitiesfor all possible antenna configurations for the first antennas and thesecond antennas.
 4. The method of claim 1, further comprising the stepof transmitting information related to the selected at least twoantennas to the home base station.
 5. The method of claim 4, whereinupon receiving an approval for use of the selected at least two antennasfrom the home base station, the mobile station after transmitting theinformation, releases connections to antennas through which current datachannels are formed, and forms new data channels through the selected atleast two antennas.
 6. The method of claim 1, wherein the channelquality includes at least one of a noise power, a signal power, acorrelation feature, a Received Signal Strength Indicator (RSSI), aCarrier-to-Interference ratio (C/I), a Signal-to-Noise Ratio (SNR), anda Signal-to-Interference Noise Ratio (SINR).
 7. A system for reselectingan antenna for a data channel by a mobile station in a cellular mobilecommunication system that includes the mobile station having at leasttwo antennas, a home base station having a plurality of antennas, atleast one of which is used to form the data channel to the mobilestation, and at least one neighbor base station for the home basestation, the system comprising: a mobile station for determining whetherto reselect antennas based on channel qualities for first antennas ofthe home base station and second antennas of the neighbor base station,selecting at least two antennas from among the first antennas and thesecond antennas based on the measured channel qualities, andtransmitting information on the selected at least two antennas to thehome base station; and a home base station for receiving information onthe measured channel qualities and transmitting a command for selectionof at least the two antennas to the mobile station according to thereceived information.
 8. The system of claim 7, wherein the mobilestation comprises: a storage for storing information on the home basestation and the first antennas, and information on the neighbor basestation and the second antennas; a parameter measurer for measuring thechannel qualities based on the information stored in the storage; achannel performance estimator for generating performance of the firstand second antennas using the channel qualities; an antenna reselectcontroller for determining whether to reselect the antennas, andselecting the at least two antennas in response to an antenna selectcommand form the home base station; and a feedback signal generator fortransmitting to the home base station the measured channel qualities andinformation on the selected at least two antennas.
 9. The system ofclaim 8, wherein the antenna reselect controller defines a thresholdvalue for the channel quality, compares a channel quality, for anantenna to which a current channel is formed, with the threshold value,and determines whether to reselect the antennas based on the comparisonresult.
 10. The system of claim 8, wherein the antenna reselectcontroller compares a channel quality for an antenna to which a currentchannel is formed, with channel qualities for all possible antennaconfigurations for the first antennas and the second antennas, anddetermines whether to reselect the antennas according to the comparisonresult.
 11. The system of claim 7, wherein the home base stationcomprises: a channel information processor for receiving the measuredchannel qualities and information on the selected at least two antennas;an antenna reselect manager for transmitting a command for the selectionof the at least two antennas to the mobile station according to themeasured channel qualities; and an antenna reselect scheduler forallocating the selected at least two antennas to the mobile station. 12.The system of claim 11, wherein if the at least two antennas include thesecond antennas, the antenna reselect manager transmits correspondinginformation to the neighbor base station.
 13. The system of claim 7,wherein the channel quality includes at least one of a noise power, asignal power, a correlation feature, a Received Signal StrengthIndicator (RSSI), a Carrier-to-Interference ratio (C/I), aSignal-to-Noise Ratio (SNR), and a Signal-to-Interference Noise Ratio(SINR).
 14. The system of claim 7, wherein upon receiving an approvalfor use of the selected at least two antennas from the home basestation, the mobile station releases connections to antennas throughwhich current data channels are formed, and forms new data channelsthrough the selected at least two antennas.